The invention relates to a process for the construction of drain system.
The purpose of the drain system, and drying rib or other structure is to protect earth banks or engineering structures against the detrimental effect of the flowing or seeping groundwater. The process according to the invention is applicable for dewatering the slipping, wet, cohesionless or cohesive soils and layers exposed to the risk of land-slide.
Drain systems, drying ribs or similar structures are built to arrest soil movements, or to anticipate expectable soil movements. These are made for two reasons, partly the drain system removes the undesirable groundwater, which will influence favourably the slope stability, and partly the friction arising on the surface of the ribs prevents the soil movement. Besides protection of the bank, insulated or poorly insulated structures are frequently protected against seeping groundwaters with the construction of drain systems.
At the same time the drain system is built by excavating the soil from the timbered pit manually, or mechanically, then concrete bed is laid at the bottom of the trench, on which a permeable medium or drain pipe is arranged. A highly permeable suction body is built above it, sealed with clay plug stamped on the ground surface. This clay plug protects the suction body against the clogging effect of the surface waters. For this purpose, occasionally, a covered surface catchwater drain is built above the drain system.
According to experiences, the above process is very expensive, demanding much manual labor and interference with the environment. For these reasons this process for cases involving 6-8 m depth is more and more avoided.
Such process for the construction of a drain system is known from Hungarian patent specification No. 178 870, where effective deep drain systems are built with dry cutting in stable, e.g. clayey soils. This process utilizes the property present in the more cohesive soils, that the walls of a 7-8 m deep pit remain stable for a short time, until the drain pipe and suction body are built in. Prior to building the drain system, such wide working area is required as to be suitable for the safe traffic of the bulldozers and transport vehicles. The drain pipes are lowered manually and with bulldozers into the trench excavated with a trench-dredger, and laid into a dry concrete bed. Then the drain pipe is covered with crushing as suction body, followed with sandy gravel covered with geotextile.
According to experience, the use of this process at a depth of 8 m is uncertain, caving-in of the pits is frequent, requiring complicated and costly restoration. Further shortcoming of the process is, that it cannot be used in depth beyond 10 m, and in cohesionless, e.g. sandy soils, or when such layers alternate.